Tensions are escalating dangerously in the South China Sea, where China and Vietnam have conflicting territorial interests. Chinese government vessels rammed Vietnamese patrol boats near an oil rig that China’s state-owned company is installing there, and shot water cannons at Vietnamese vessels.

This is a very serious situation. China had no right to unilaterally start such an economic undertaking in disputed waters in the first place. The Chinese side must desist immediately.

China describes the Paracel Islands, located near the site of the maritime skirmish, as part of its territory and claims it has every right to drill for oil in waters around the islands.

But the entire area falls within the exclusive economic zone established by Vietnam. Although the Paracel Islands have been under China’s effective control, they are also claimed by Vietnam. China’s behavior is unacceptable.

The incident marks yet another chapter in the troubled history of territorial disputes in the South China Sea. In addition to China and Vietnam, the Philippines, Malaysia, Brunei and Taiwan claim islands and economic interests in the region.

For its part, China claims most of the South China Sea, encompassed in what is called the “nine-dotted line.” But this U-shaped demarcation line is vague in nature and not clearly based on international law. Beijing seems to believe it holds exclusive rights in the waters within the line.

In January, the government of China’s Hainan province, which Beijing says has jurisdiction over the South China Sea, introduced a set of rules that require all foreign fishing boats to gain permission from provincial authorities to operate in the vast area claimed by China. The move, unsurprisingly, provoked angry protests from neighboring countries. It appears that China is trying to accumulate a number of faits accomplis concerning its territorial claim over the area within the nine-dotted line.

China and Vietnam are both communist countries, but the history of their relationship is checkered, as shown by the 1979 Sino-Vietnamese War. Even so, in the 1990s, the two countries devoted considerable time to working together to resolve disputes over their land border and maritime rights in the Gulf of Tonkin. They eventually struck a deal.

At that time, the two countries failed to reach an agreement on their territorial claims over the South China Sea. But the deal was viewed as sage move and a way of avoiding conflict that would harm bilateral ties.

In 2002, China and the Association of Southeast Asian Nations (ASEAN) agreed on the Declaration on the Conduct of Parties in the South China Sea, a set of principles for peaceful resolutions to disputes in the region. Both sides are set to work out a detailed code of conduct to achieve the goal outlined in the agreement. China’s latest action tramples on these international agreements.

The U.S. government quickly expressed its concern about this latest flare-up. Washington regards China’s behavior as a potentially serious threat to the principle of freedom of navigation in the entire South China Sea.

If things don’t improve, the seas in East Asia will become theaters of conflict among regional powers. That would be in nobody’s interest. China, which is responsible for this situation, should back down first. We also urge Vietnam to keep calm in responding to the situation.